Re: [Qemu-devel] [PATCH v3 3/7] migration: Wait for semaphore before completing migration

[Peter Xu <peterx@redhat.com>]

On Wed, Oct 18, 2017 at 06:40:09PM +0100, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> 
> Wait for a semaphore before completing the migration,
> if the previously added capability was enabled.
> 
> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> ---
>  migration/migration.c | 38 ++++++++++++++++++++++++++++++++++++++
>  migration/migration.h |  3 +++
>  2 files changed, 41 insertions(+)
> 
> diff --git a/migration/migration.c b/migration/migration.c
> index 86ae0292f0..6c6d5e2c75 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -1968,6 +1968,39 @@ fail:
>  }
>  
>  /**
> + * migration_maybe_pause: Pause if required to by
> + * migrate_pause_before_switchover called with the iothread locked
> + * Returns: 0 on success
> + */
> +static int migration_maybe_pause(MigrationState *s, int *current_active_state)
> +{
> +    if (!migrate_pause_before_switchover()) {
> +        return 0;
> +    }
> +
> +    /* Since leaving this state is not atomic with posting the semaphore
> +     * it's possible that someone could have issued multiple migrate_continue
> +     * and the semaphore is incorrectly positive at this point;
> +     * the docs say it's undefined to reinit a semaphore that's already
> +     * init'd, so use timedwait to eat up any existing posts.
> +     */
> +    while (qemu_sem_timedwait(&s->pause_sem, 1) == 0) {
> +        /* This block intentionally left blank */
> +    }

Not sure whether worth generalizing this (along with the comment) into
a function like qemu_sem_consume_all(), but I'm fine with either way,
especially if no respin is planned.

(The comment is not really migration specific as well - IMHO it's a
 general problem for many other semaphores)

Reviewed-by: Peter Xu <peterx@redhat.com>

> +
> +    qemu_mutex_unlock_iothread();
> +    migrate_set_state(&s->state, *current_active_state,
> +                      MIGRATION_STATUS_PRE_SWITCHOVER);
> +    qemu_sem_wait(&s->pause_sem);
> +    migrate_set_state(&s->state, MIGRATION_STATUS_PRE_SWITCHOVER,
> +                      MIGRATION_STATUS_DEVICE);
> +    *current_active_state = MIGRATION_STATUS_DEVICE;
> +    qemu_mutex_lock_iothread();
> +
> +    return s->state == MIGRATION_STATUS_DEVICE ? 0 : -EINVAL;
> +}
> +
> +/**
>   * migration_completion: Used by migration_thread when there's not much left.
>   *   The caller 'breaks' the loop when this returns.
>   *
> @@ -1993,6 +2026,9 @@ static void migration_completion(MigrationState *s, int current_active_state,
>              bool inactivate = !migrate_colo_enabled();
>              ret = vm_stop_force_state(RUN_STATE_FINISH_MIGRATE);
>              if (ret >= 0) {
> +                ret = migration_maybe_pause(s, &current_active_state);
> +            }
> +            if (ret >= 0) {
>                  qemu_file_set_rate_limit(s->to_dst_file, INT64_MAX);
>                  ret = qemu_savevm_state_complete_precopy(s->to_dst_file, false,
>                                                           inactivate);
> @@ -2373,6 +2409,7 @@ static void migration_instance_finalize(Object *obj)
>  
>      g_free(params->tls_hostname);
>      g_free(params->tls_creds);
> +    qemu_sem_destroy(&ms->pause_sem);
>  }
>  
>  static void migration_instance_init(Object *obj)
> @@ -2383,6 +2420,7 @@ static void migration_instance_init(Object *obj)
>      ms->state = MIGRATION_STATUS_NONE;
>      ms->xbzrle_cache_size = DEFAULT_MIGRATE_CACHE_SIZE;
>      ms->mbps = -1;
> +    qemu_sem_init(&ms->pause_sem, 0);
>  
>      params->tls_hostname = g_strdup("");
>      params->tls_creds = g_strdup("");
> diff --git a/migration/migration.h b/migration/migration.h
> index 969866303e..cd988a99b9 100644
> --- a/migration/migration.h
> +++ b/migration/migration.h
> @@ -121,6 +121,9 @@ struct MigrationState
>      /* Flag set once the migration thread called bdrv_inactivate_all */
>      bool block_inactive;
>  
> +    /* Migration is paused due to pause-before-switchover */
> +    QemuSemaphore pause_sem;
> +
>      /* The semaphore is used to notify COLO thread that failover is finished */
>      QemuSemaphore colo_exit_sem;
>  
> -- 
> 2.13.6
> 

-- 
Peter Xu